A starter-generator in a motor vehicle can in principle be operated at a vehicle electric system voltage of 14 V (14 V or, more precisely, 14.4 V is the charging voltage of a 12 V battery). It is technically expedient in this respect to limit the power output and also the generator output to no more than 3 kW, as otherwise the vehicle electric system currents would be too high. A starter-generator can start the internal combustion engine and supply the electric loads during the journey at a vehicle electric system voltage of 14 V, but an output of more than 3 kW is required for additional functions such as boost (accelerate) or recuperation (braking). This output can only be realized with a higher vehicle electric system voltage. Consequently 42 V vehicle electric systems (42 V is the charging voltage of a 36 V battery) that permit greater electrical outputs are being developed.
An integrated starter-generator, abbreviated to ISG, is, for example, a three-phase asynchronous motor with electronic inverter, which asynchronous motor is connected directly to the crankshaft of the internal combustion engine at the flywheel and enables the generation of electrical energy in generator mode and the generation of mechanical motive power in motor mode. The three-phase asynchronous motor thus replaces both the known generator and the known starter. The available outputs increase considerably (6 kW as opposed to 2 kW with known generators), so the ISG enables extra functions in addition to starting the engine and supplying the vehicle electric system:    Boost (accelerate): Torque support for the internal combustion engine during the vehicle acceleration phase. The ISG offers a maximum torque of around 200 Nm, which is approximately equal to the torque of a 2-liter internal combustion engine. Currents of up to 270 A are required for around 15 seconds during the boost operation;    Recuperation (regenerative braking): The ISG dissipates kinetic energy during braking of the vehicle by generating electricity. Currents of up to 270 A are generated in this process for up to 30 seconds.
Model calculations indicate a saving on fuel of up to 20% when these new functions are used.
One major problem is the cost-effective implementation of an energy storage mechanism able to manage the high charging and discharging currents over the long term. It is estimated that there will be between 300,000 and 500,000 charging and discharging cycles over the lifetime of a motor vehicle (approximately 150,000 km).
Known 12 V motor vehicle electric systems are equipped with lead-acid batteries. This type of battery has a very limited cycling resilience, as the method it uses to transport energy, namely a transport of material (ions), entails notable losses. What this means is that the structure of the electrodes changes continuously as the active surface shrinks (loss of capacity) and material is lost. This loss of material leads to the deposition of lead sludge.
The batteries in the 14 V vehicle electric system are at the present time operated at an almost constant battery charge. The brief periods of current consumption for starting and idling are virtually insignificant. The principal function of the battery is effectively that of a substantial buffer capacitor that stabilizes the vehicle electric system thanks to its large capacity. The battery loses capacity only slowly and accordingly has a lifetime of many years.
The situation in the 42 V vehicle electric system with highly dynamic operation at the ISG is quite different. Model calculations suggest that a typical lead-acid battery can be expected to last for between 5,000 and 10,000 km. An empirical trial found that the battery survived a journey distance of 6,000 km before failing. It is also technically difficult to ensure charging capacity for currents of up to 270 A at all times. The result of this is that the battery must be replaced as part of every scheduled service of the vehicle and possibly even between scheduled services if lead-acid batteries are used for boost and recuperation. This is completely unacceptable.
A system with a 36 V lead-acid battery does have the lowest initial cost by some distance, but the follow-up costs that ensue over the lifetime of the vehicle are very high.
NiMH (nickel metal hydride) batteries are also suitable in principle for boost and recuperation, but the battery has to be significantly over-sized in order to achieve the requisite cycling resilience. If calculations show that a battery of 6 kW and 11 Ah is required to provide the energy or output needed, at least 14 Ah will be required to achieve the requisite cycling resilience. Unresolved problems include the issue of how to dissipate the heat generated by major charge replacement. It would not, moreover, be possible to make any significant saving on the initial costs due to the materials used.
Li-Ion (lithium ion) batteries promise much from a technical standpoint (energy density, weight, efficiency, etc.). Development work is already underway, but it will be several years before products suitable for use in automobiles become available. The costs identifiable today suggest that using Li-Ion batteries could be even more expensive than using NiMH batteries.
Only a few energy storage mechanisms are able to provide the required high number of cycles and high energy throughput (200,000 boost operations demand a total of around 12.6 MWh of energy or approximately 180,000 Ah). Double layer capacitors (DLC) are a suitable energy storage mechanism. Double layer capacitors are already available and are able to store and release the energy converted in boosting and recuperation. They are also able to manage the currents encountered in these operations without difficulty and their good efficiency means that self-heating is low.
The limited energy storage capacity of the double layer capacitor does, however, mean that an additional battery, for example a low cost lead-acid battery, is required. The lead-acid battery will not be exposed to cycling loads in this configuration and should therefore have a lifetime in line with the present standard.
A starting operation of 1 second draws approximately 2 Wh of energy from the energy storage mechanism. Statistical calculations indicate two starting operations per kilometer driven. Similarly, two boost operations are expected per kilometer driven when accelerating. A total of up to 63 Wh can be required here per boost operation with a 6 kW starter-generator. The maximum current in the starting operation can exceed 500 A and the maximum current in the boost operation can be more than 250 A.
It is disadvantageous, however, that energy can in principle be exchanged at the capacitor only by means of voltage variation. The capacitor voltage must be varied by 50% in order to move 75% of the charge (E=½ C*(U22−U12)). This would entail an unacceptable variation from 42 V (in the fully charged state) to 21 V for the 42 V vehicle electric system. Known designs consequently use a bi-directional DC/DC converter between the vehicle electric system and the double layer capacitor DLC to compensate for this shortcoming. This makes it possible to keep the 42 V vehicle electric system stable while the voltage at the double layer capacitor varies.
The combination of double layer capacitor and lead-acid battery is presently the most promising solution from the technical standpoint. The double layer capacitor and battery combination is expensive, however, and so is the bi-directional DC/DC converter, so the total cost of such a solution is likely to be considerable. The high costs involved have a number of effects, not least among which is that they delay the rapid implementation of this system in series-produced vehicles.